The present invention relates to a light source utilizing a microwave generated plasma discharge.
Recently, a light source utilizing high frequency discharge, particularly, a microwave generated plasma discharge, has been considered in view of the long life thereby provided, which is significantly longer than the life of a conventional light source having electrodes which are relatively easily consumed.
A light source using high frequency discharge has essentially no electrode and thus there is no thermal loss such as is inherent to a light source having electrodes. Further, the discharge impedance thereof at the time when discharge starts is not significantly different from that during the stable discharge. In addition to these advantages, since discharge power is localized around an envelope of the lamp bulb, it is easy to couple power to the light source at the discharge starting time. Thus, the time required to achieve the maximum lamp output is short.
FIG. 1 shows, in vertical cross section, a conventional microwave generated plasma light source constructed by incorporating the above features and FIG. 2 is a cross section of the light source taken along a line II--II in FIG. 1.
In these Figures, a magnetron 1 enclosed by an envelope 10 and cooled by a cooling fan 7 generates microwave energy which is radiated through a magnetron antenna 2 into a waveguide tube 3. The microwave energy propagates along the waveguide tube 3 and is radiated through a feeding opening 5 to a cavity 49 having a semicircular cross section and defined by a mesh 9 and a semicircular light reflector 4 having a plurality of gas passages formed therein to thus establish a microwave electromagnetic field therein. A discharge occurs in a noble gas encapsulated in a discharge bulb 6 due to the microwave electromagnetic field to thus heat the bulb wall or envelope and to thereby evaporate a metal such as mercury also encapsulated in the bulb. Then, discharge in the gaseous metal takes place. With this gaseous metal discharge, the microwave energy is caused to be absorbed by the discharge bulb 6 substantially completely during its propagation along the length of the discharge bulb 6 through several reflections within the cavity 49 so that the microwave energy is converted into discharge energy substantially completely. That is, the bulb is excited in a non-resonance state.
The reflector 4 defining a portion of the cavity 49 reflects light directed rearwardly of the lamp bulb so that all the light from the bulb is directed to pass through an open end of the cavity which is covered by a mesh member 9 which is transparent to light but only translucent to microwaves to thereby utilize the light produced by the gaseous metal discharge effectively.
Cooling air supplied by the fan 7, after cooling of the magnetron 1, passes through the air passages 8 of the reflector 4 to cool the discharge bulb 6 and is discharged from the cavity 49 through the mesh member 9.
In the conventional microwave generated plasma light source constructed as above, the microwave electromagnetic waves are distributed in the cavity 49 having a semicircular cross section as shown in FIG. 3. However, the distribution is not uniform. Therefore, the discharge in the discharge bulb 6 is not uniform and thus the light intensity distribution is not uniform in the axial direction of the bulb.
One approach of eliminating the non-uniformity of light intensity distribution is to alter the shape of, for example, the reflector 4. This approach, however, is not practical because it is difficult as a practical matter to provide a reflector of a shape corresponding to the microwave electric field distribution in the cavity. Another approach is to use as small a discharge bulb as possible to thereby obtain a uniform discharge. Since, in this case, however, the cavity 49 is used in the non-resonance state, it is impossible to supply sufficient power to the discharge bulb 6 to excite it resulting in a low discharge efficiency. Therefore, the device thus constructed is not suitable for use as an ultraviolet ray source for photographic plate making where a high light intensity and a highly uniform illumination distribution over an area to be illuminated are required.